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MINE ID # OR PROSPECTING ID#: M-1987-064 PAGE: 2 INSPECTION DATE: 9-8-2004 INSPECTOR'S INITIALS: ACS OBSERVATIONS This inspection was conducted as part of the Division of Minerals and Geology's (DMG) review of pending permit amendment AM-001. AM-001 includes a proposal to excavate a pit in the east overbank of the Animas River. The edges of the pit will be no closer than 300 feet to the existing top of the left bank of the river. A report prepared by Southwest Hydro-Logic and submitted to DMG in support of the AM-001 application indicates that 100-year flood flows in the Animas will not overtop the east riverbank. The DMG would like to compare the conclusions in the Hydro-Logic report with the floodplain shown on the National Flood Insurance Program (NFIP) maps for this location, and requests that the Operator provide the pertinent NFIP maps for DMG review. The Flood Hazard Area Delineation or other such reports that form the basis for the NFIP mapping would also be helpful to the DMG in the review of the floodplain issues as they may relate to the proposed pit. With the proposed pit being outside the floodplain, the potential for excavation of the pit to have an adverse offsite impact during or after a large flood is greatly diminished. However, if the east bank of the river were to cut toward the pit as a result of high flows, the minimum 300-foot setback from the existing riverbank would be diminished, and if rapid bank cutting were to occur, flood flows could spill into the proposed pit. However, several sections of the east bank of the river have been armored by the Operator to prevent bank cutting. This armor is likely to be highly effective in preventing erosion during annual snowmelt flooding of the Animas River. These annual floods vary in duration and intensity depending on the snowpack. The peak flows from snowmelt flooding, even in heavy snow years, are much less than peak flows during a 100-year flood. The duration of flooding during snowmelt is much longer than for a 100-year flood. These differences between the annual snowmelt flood and the 100-year flood are due to the nature of the 100-year flood in the Animas River basin. The 100-year flood would be caused not by snowmelt, but by intense and persistent rainfall during a frontal boundary storm that would be most likely to occur during late summer or autumn. There has not been a 100-year flood of the Animas River in recorded history, and observation of the effectiveness of bank protection during annual snowmelt flooding, while useful, is an imperfect gauge of how the armor would function during a 100-year flood. The most likely reach of the bank that could cut in the direction of the pit is at the north end of the permit area. Through this reach the Operator has placed stepped or battered tiers of large concrete blocks 32-inches thick x 3- feet wide x 8-feet long. There are several photos of the block armoring included in this report. The blocks are too large to be transported by 100- year flood flows. However, the base of the tiers of blocks is located above the elevation of the deepest part of the river channel. As a result, the blocks could be undercut during a very large flood causing them to topple into the floodwaters. Even after toppling, the blocks are large and heavy enough to resist rapid cutting into the bank. It is the DMG's understanding that bank armoring at this site was approved,